1. Field of the Invention
The present invention relates to a method of machining a spherical surface, and more particularly to a non-contact machining method suitable for abrading or polishing a small-diameter lens such as a rod lens.
2. Description of the Relevant Art
Recent efforts to achieve higher performance of small-diameter rod-shaped graded-index glass lenses include machining one end of such a rod lens into a spherical surface.
According to one conventional process for machining one end of a rod lens, a spherical surface is created on the end of the rod lens with a curve generator. The principles of such a curve generator are shown in Optical Technology Contact, Vol. 24, No. 10 & No. 12 (1986) and Vol. 25, No. 2 & No. 6 (1987), for example.
The conventional machining process will be described below with reference to FIGS. 6 through 9 of the accompanying drawings. As shown in FIG. 6, a grinding wheel 302 of a large grain size (low grain number) which is mounted on a tip end of a cup 301 is rotated about its own axis, and a rod lens 2 mounted on a blocking tool 24 by an adhesive 23 is positioned such that its axis 22 passes through the center P of a round surface of the grinding wheel 302. Then, the rod lens 2 is pressed against the rotating grinding wheel 302, with a fluid lubricant-coolant supplied from a pipe 303 into the region where the rod lens 2 contacts the grinding wheel 302.
If the lens axis 22 is inclined with respect to the axis 315 of the grinding wheel 302 at an angle of .beta., then a spherical surface having a radius of curvature which is indicated by R=(D/2 sin .beta.)-r is generated on the tip end of the rod lens 2, where D is the diameter of the grinding wheel 302 and r is the radius of curvature of the round surface of the grinding wheel 302.
Then, the generated spherical surface of the rod lens 2 is abraded so that the surface is corrected in shape and improved in surface roughness. More specifically, as shown in FIG. 7, an abrading tool 401 comprising a grinding wheel having a concave spherical surface is placed on the spherical surface of the rod lens 2. The lower end of a vertical shaft 402 which is supported on the distal end of an arm 407 (FIG. 8) of a known abrading machine 406 is coupled to the center of an upper surface of the abrading disc 401 by a ball-and-socket joint. The vertical shaft 402 is then laterally oscillated back and forth about the center Q of the concave spherical surface of the abrading disc 401. A fluid lubricant-coolant is supplied from a pipe 403 into the abrading disc 401, and the rod lens 2 is rotated about the axis 22 and pressed against the abrading disc 401 under a prescribed pressure. The spherical surface of the rod lens 2 is thus abraded for surface finishing. Abrading tools of progressively smaller grain sizes (higher grain number) are successively used on the abrading machine 406, so that the spherical surface of the rod lens 2 is corrected in shape and improved in surface roughness through successive abrading steps for finer surface finishes.
Thereafter, the abraded spherical surface of the rod lens 2 is subjected to surface polishing. More specifically, as shown in FIG. 9, a polishing tool 501 having a concave spherical surface with a resilient layer 502 of urethane, tar, or the like applied is placed on the spherical surface of the rod lens 2. The polishing tool 501 and the rod lens 2 are then moved in the same manner as described above, while a fluid lubricant-coolant is being supplied from a pipe 503 into the blocking tool 501. The spherical surface of the rod lens 2 is thus polished.
With the conventional machining process, however, the concave spherical surfaces of the abrading and polishing tools are susceptible to deformations due to mutual lapping. It requires highly skilled hands to remove such deformations from the abrading and polishing tools. Therefore, the quality of finished rod lenses is not assured at a stable level.
The lens abrading process involves a number of successive steps for progressively finer surface finishes. The abrading tools used are designed for a particular radius of curvature of the spherical surface of a rod lens. If a rod lens having a spherical surface with a different radius of curvature is to be abraded, the spherical surfaces of the abrading tools must be corrected. Since the correcting process is tedious and time-consuming and needs a number of steps, the rate of production of rod lenses is low.